Mixed metal oxides represent a large class of materials with many important properties not found in the more limited binary oxides. Mixed metal oxides can stabilize high oxidation states of transition elements, have extensive ranges of either anion or cation nonstoichiometry and provide new structural arrangements such as found in the perovskite, spinel or pyrochlore structures. These features made mixed metal oxides useful for applications such as electrocatalysts, heterogeneous catalysts and battery cathodes.
This flexible class of compounds is limited in application because of the difficulties involved in the preparation of high surface area materials. The usual route to these compounds provides low surface area material because it involves reaction at high temperature for long periods of time to overcome the diffusional limitations of compound formation.
The traditional ceramic approach to these complex metal oxides involves repeated high temperature firing of the component oxides with frequent regrindings. This harsh treatment is necessary to otain a single phase product because of the diffusional limitation of reaction even between fine particles of 10.mu. size (100,000 A). Initial reaction is rapid but further reaction goes more and more slowly as product builds up and diffusion paths become longer. The use of freeze drying or coprecipitation improves reactivity of the component oxides because these techniques give initial particles of 0.05.mu. (500 A) that are well mixed. But 500 A particles mean that diffusion must still occur across more than 50 unit cells, because each of these particles has the composition of one of the component oxides. To achieve the fastest, most complete reaction in the shortest time would require the mixing of the components of an atomic scale.
German Patent 1,342,020 discloses a method for preparing catalysts and catalyst carriers consisting of metal oxides, of metals and metal oxides or of metals and/or metal oxides and carrier which contain the component(s) in a very finely divided and difficultly recrystallizable form. Such catalysts may be advantageously produced by a process which comprises mixing at least two metal salts in aqueous solution in a given proportion and precipitating basic carbonates having the general formula A.sub.6 B.sub.2 (OH).sub.16 (CO.sub.3).4H.sub.2 O by means of an aqueous solution of an alkali bicarbonate at a temperature of from 50.degree. to 100.degree. C, drying the resulting precipitate and calcining, or calcining and reducing the dried product to obtain high surface area metal oxides. However, their system is limited and specific to basic carbonates (i.e. containing hydroxides) that are hydrated and of the formula given above. In that formula, A is a small divalent cation and B is a trivalent cation. Both the A and B site can be occupied by any fraction of different cations as long as the divalent ones (A) represent 75% of all cation sites and the trivalent ones represent 25% of all cation sites.
By comparison, the instant specification discloses a process for the preparation of high surface area mixed metal oxides which involves the precipitation of pure carbonate (only CO.sub.3.sup..dbd. ions) and not a basic carbonate, the precursor of the instant invention does not contain any waters of hydration (whereas German 1,342,020 does so), there is only one type of cation site in the instant carbonate precursor (ACO.sub.3) (a divalent cation), no trivalent cations as in German 1,342,020, and the precursor of the instant specification is able to accommodate large cations such as Ca, and Cd while German 1,342,020 is limited to smaller cations. From this it is clear that the precursors of the instant specification are in every way different than those of the prior art (in the basic carbonates of the German Patent).
U.S. Pat. No. 2,275,181 to Ipatieff and Corson teaches the preparation of catalysts by the coprecipitation of copper carbonate or copper hydroxide with a hydroxide of zinc or iron by the addition of ammonium carbonate solution to a solution containing the desired proportions of the nitrates of copper and of the desired promoter metal (zinc or iron). The precipitate obtained is calcined at from 350.degree.-450.degree. in air and then reduced in H.sub.2 at 175.degree.-200.degree. C. The use of copper operates against the formation of a carbonate possessing the calcite structure and, therefore, renders impossible the very specific structure necessary to obtain the compounds possessing the specific inter atomic distances and distributions which are unique characteristics of compounds prepared by the instant process.
U.S. Pat. No. 3,303,001 to Dienes also teaches a copper-zinc catalyst. The catalyst is prepared by the coprecipitation of copper and zinc as the carbonates from an aqueous solution of their soluble salts through double decomposition reaction with sodium carbonate. Again, the procedure results in a carbonate which does not possess the calcite structure (since copper is incapable of entering a crystal lattice possessing the calcite configuration). The copper carbonate precipitates out as a separate crystalline phase and does not enter into the same intimate relationship as do the metals outlined in the instant specification. This behavior by the copper in forming a separate phase is designated as coprecipitation technique and clearly is not the solid solution of the instant invention.